JAK kinases (JAnus Kinases) are a family of cytoplasmic protein tyrosine kinases including JAK1, JAK2, JAK3 and TYK2. Each of the JAK kinases is selective for the receptors of certain cytokines, though multiple JAK kinases may be affected by particular cytokine or signaling pathways. Studies suggest that JAK3 associates with the common gamma (γc) chain of the various cytokine receptors. JAK3 in particular selectively binds to receptors and is part of the cytokine signaling pathway for IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21. JAK1 interacts with, among others, the receptors for cytokines IL-2, IL-4, IL-7, IL-9 and IL-21, while JAK2 interacts with, among others, the receptors for IL-9 and TNF-α. Upon binding of certain cytokines to their receptors (for example, IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21), receptor oligomerization occurs, resulting in the cytoplasmic tails of associated JAK kinases being brought into proximity and facilitating the trans-phosphorylation of tyrosine residues on the JAK kinase. This trans-phosphorylation results in the activation of the JAK kinase.
Phosphorylated JAK kinases bind various STAT (Signal Transducer and Activator of Transcription) proteins. STAT proteins, which are DNA binding proteins activated by phosphorylation of tyrosine residues, function both as signaling molecules and transcription factors and ultimately bind to specific DNA sequences present in the promoters of cytokine-responsive genes. JAK/STAT signaling has been implicated in the mediation of many abnormal immune responses such as allergies, asthma, autoimmune diseases such as transplant (allograft) rejection, rheumatoid arthritis, amyotrophic lateral sclerosis and multiple sclerosis, ocular disorders and diseases, as well as in solid and hematologic malignancies such as leukemia and lymphomas.
Connective tissue disorders are a group of diseases that affect a broad variety of organs, and are sometimes referred to as collagen vascular diseases. This class of diseases includes many distinct inflammatory disorders, such as vasculitis, discoid lupus erythematosus (DLE), systemic lupus erythematosus (SLE), progressive systemic sclerosis, polymyositis/dermatomyositis, polymyalgia rheumatic, polyarteritis nodosa, and Wegener's granulomatosis. These are distinct diseases that have been clinically distinguished from each other for purposes of diagnosis, prognosis, and treatment.
Lupus erythematosus is a generic category of disease that includes both systemic and cutaneous disorders. The systemic form of the disease can have cutaneous as well as systemic manifestations. However, there are also forms of the disease that are only cutaneous without systemic involvement. For example, SLE is an inflammatory disorder of unknown etiology that occurs predominantly in women, and is characterized by articular symptoms, butterfly erythema, recurrent pleurisy, pericarditis, generalized adenopathy, splenomegaly, as well as CNS involvement and progressive renal failure. The sera of most patients (over 98%) contain antinuclear antibodies, including anti-DNA antibodies. High titers of anti-DNA antibodies are essentially specific for SLE. Conventional treatment for this disease has been the administration of corticosteroids or immunosuppressants.
There are three forms of cutaneous lupus: chronic cutaneous lupus (also known as discoid lupus erythematosus or DLE), subacute cutaneous lupus, and acute cutaneous lupus. DLE is a disfiguring chronic disorder primarily affecting the skin with sharply circumscribed macules and plaques that display erythema, follicular plugging, scales, telangiectasia and atrophy. The condition is often precipitated by sun exposure, and the early lesions are erythematous, round scaling papules that are 5 to 10 mm in diameter and display follicular plugging. DLE lesions appear most commonly on the cheeks, nose, scalp, and ears, but they may also be generalized over the upper portion of the trunk, extensor surfaces of the extremities, and on the mucous membranes of the mouth. If left untreated, the central lesion atrophies and leaves a scar. Unlike SLE, antibodies against double-stranded DNA (eg., DNA-binding test) are almost invariably absent in DLE.
Conventional DLE treatments have included topical corticosteroid ointments or creams, such as triamcinolone acetonide, fluocinolone, flurandrenolide, betamethasone valerate, or betamethasone dipropionate. Resistant plaques can be injected with an intradermal corticosteroid. However, prolonged use of corticosteroids themselves can lead to serious side effects, such as skin atrophy, striae, easy bruising and tearing of the skin, dermatitis, telangiectasia, and increased susceptibility to infection. Other potential DLE treatments include calcineurin inhibitors such as pimecrolimus cream or tacrolimus ointment. Particularly resistant cases can be treated with systemic antimalarial drugs, such as hydroxychloroquine (PLAQUENIL). However, this drug carries the risk of significant retinal toxicity with bull's eye retinopathy. Even after cessation of the drug, visual loss may continue and no medical therapy has been found to reverse the retinal damage.
DLE is a disfiguring disease for which current therapies have proven unsatisfactory. Treatments are also needed for the other cutaneous forms of lupus, such as the acute and sub-acute forms.